This is a Phase II multicenter, randomized, double-blind, multiple-dose, dose-escalation study to evaluate the tolerability, pharmacokinetics, biological activity and efficacy of BG9273 (Biogen Inc.'s LFA-3/IgG1 Fusion Protein) when administered as multiple doses to patients with moderate to severe, chronic psoriasis. Each patient will receive one 30 minute intravenous infusion of BG9273 every week for a total of eight doses and will be assessed by clinical and laboratory examination for evidence of toxicity and biologic response.